Biotin (vitamin H) is a readily water-soluble substance found at low concentrations in blood and tissues. The biological role of biotin is as a carrier of activated CO2 and it permits the transfer of CO2 to acceptors without the need for additional free energy. The activated carboxybiotin is usually attached to an enzyme that is required for the formation of carboxybiotin. For example, biotin may be attached to pyruvate carboxylase which, in the presence of acetyl CoA, catalyses the formation of carboxybiotin and the subsequent transfer of the activated carboxyl group to pyruvate, to form oxaloacetate.
Biotin also binds with one of the highest naturally known affinities to avidin, a 63 kDa glycoprotein from chicken egg white, and to streptavidin, a non-glycosylated protein from the bacterium Streptomyces avidinii. The binding is almost irreversible in character (Ka 1015 mol−1). The affinity between avidin and biotin has proved very useful in a wide variety of bioanalytical applications. For example, the avidin-biotin complex has been used successfully in a wide variety of detection systems where target molecules are combined with biotin through its carboxy terminus, to form biotinylated molecules which may be easily detected or separated from solution. Biotinylation can occur without changing the biological or physiochemical properties of the various molecules and without affecting the binding capacity of the biotin prosthetic group to avidin.